Chemically and structurally functionalized graphene for real-world applications

After ten years of discovering the amazing properties of pristine graphene and seeking its uses in electronics, photonics, and novel technologies, researchers are starting to add another layer to the beautifully complex world of graphene: chemically functionalized and nanostructured graphene.

The use of graphene for water desalination is back to the spotlight, after recent reports on controllable manufacture of nanopores in graphene films. Last April we wrote about a theoretical simulation, done at MIT, which showed that graphene with pores having a ~1 nanometer diameter would act as an excellent water filter, passing clean water while leaving behind any chemical residue. The news of MIT's discovery was so big that the Smithsonian magazine, the publication of the famous Smithsonian Institution, named it one of the top 5 surprising scientific milestones of 2012 Nanometer-sized pores in graphene films are not only considered for water filtration, but also for rapid DNA sequencing, given that the lateral size of a DNA strand is about one nanometer.

Now, less than a year later, we are seeing first reports of sub-nanometer sized holes made controllably in graphene. At the beginning of February, we reported on our recent publication, together with Harvard University, the Lawrence Berkeley National Lab and FEI Corporation, in which we show movies of the creation of pores in a graphene film. The pores are catalyzed by residual silicon atoms and range in size from several angstroms to a few nanometers. The placement of the silicon atoms occurs randomly. Researchers at MIT have taken technology even further, showing that they can place nanopores at desired locations on the sheet. The nanopores are initiated by bombarding the sheet with gallium ions from a focused ion beam. The location of the bombardment target can be placed with nanometer accuracy, leading to graphene sheets with 5 trillion pores per square centimeter, each placed deterministically, all being of similar size.

Figure: MIT researchers bombard graphene sheets with ions, crafting a set of nanopores. Copyright: American Chemical Society.

Both papers were published in the journal Nano Letters, and both papers make use of high-resolution charged-particle microscopy. The work which Graphenea coauthored uses high-resolution transmission electron microscopy to initiate the chemical catalysis, whereas the work performed at MIT makes use of a focused ion beam. It seems that using graphene in such microscopes is gaining popularity, not only for fabrication of small holes, but also for aiding imaging. At the beginning of February, a report in the journal Advanced Materials showcased a novel use of graphene. Scientists at Michigan Technological University and the University of Illinois Chicago used graphene to encapsulate a drop of liquid, enabling the first electron-beam imaging of liquids. The ability to image liquids at the extreme resolution provided by an electron-beam microscope will have strong implications for biology and medicine, which so far relied on imaging frozen and thinly sliced biological samples.

Other recent research points out the importance of chemically functionalized and nanostructured graphene. Notably, researchers in Vienna have shown that Calcium-doped graphene can have superconducting properties. While the predicted critical temperature of 1.5 Kelvin does not set any world records (fullerenes, a carbon cousin of graphene, exhibit a critical temperature of 33K), researchers are confident that the ease of doping and local control of graphene sheets will provide additional flexibility compared to other superconducting materials. Researchers at the Brookhaven National Lab, on the other hand, studied the intercalation of Cesium atoms in graphene. They found that the intercalation is adjusted by the short-range van der Waals interaction, with the dynamics governed by defects anchored to graphene wrinkles. Then, by controlling intercalation, the scientists were able to create p-n junctions at desired locations in graphene. P-n junctions are integral parts of electronic components such as diodes and transistors, as well as some photodetectors. Also, well-defined nanoscale ferromagnetic islands underneath the graphene were built.

Finally, the Journal of Physics D published a special issue on commercialization of graphene. The edition, appropriately entitled “Graphene: from functionalization to devices”, once again underlines the importance of chemical and structural functionalization of graphene as it transforms from the amazing lab material to an enabling component of novel technologies.